Two security officers involved in an incident in which a passenger was dragged off a United Airlines flight after refusing to give up his seat have been fired, the Chicago Department of Aviation has said.

Two security officers involved in an incident in which a passenger was dragged off a United Airlines flight after refusing to give up his seat have been fired, the Chicago Department of Aviation has said.

One of the fired officers, a sergeant, was also part of an attempt to cover up some details of the incident that happened in April at Chicago's O'Hare International Airport, according to a report by the city's Office of Inspector General.

Four officers were involved in the confrontation with passenger David Dao, who was aboard a flight to Louisville, Kentucky.

The aviation department suspended the two other officers - one for five days and the other for two. The officer who received the five-day suspension resigned.

The Office of Inspector General said in the report that as a result of the office's findings and recommendations, the department fired an officer who "improperly escalated the incident" and a sergeant involved in removing facts from a report.

The report says the investigation by the Office of Inspector General found three security officers and a sergeant used excessive force and "made misleading statements and deliberately removed material facts from their reports" on the April 9 incident.

"The use of excessive force caused the passenger to hit his face on an armrest, resulting in a concussion, a broken nose, and the loss of two teeth," the report said.

Mr Dao, a physician who was 69 at the time of the incident, reached an undisclosed settlement with United less than three weeks after the incident.

The incident aboard United Express Flight 3411 was videotaped by other passengers and widely shared online. It became an international embarrassment for the airline and the aviation department.

The department also confirmed that a review of its policies and procedures is under way and will be finished by the first quarter of 2018.

Mr Dao's lawyer, Thomas Demetrio, told The Associated Press on Tuesday that they were not expecting the dismissal of the security officer who is not a sergeant, but said it may resonate with others.

"In firing him, perhaps it will send a clear message to police and airline personnel all over the world that unnecessary violence is not the way to handle passenger matters," Mr Demetrio said.

He said it surprised him that the department's review of its policies and procedures is not complete.

"That should have been done the next day," the lawyer said. "It seemed to me the policies and procedures would have been tweaked the next day."